I agree with Bruce about Haven �t you heard people using the word �serendipity � ? being the best option.
However I found a link that claims that mightn �t is used in Australian English.
http://www.usingenglish.com/forum/threads/146790-mayn-t-mightn-t.
If this is true, then we could say, "Mightn �t you have heard people using the word �serendipity � ?" which to me sounds like the speaker is trying to jog the receiver �s memory, as to whether he may have in actual fact actually heard the word �serendipity � spoken afterall.
I would love to hear from Aussies about this. p.s. I ope the word �Aussies � is politically correct and respectful when used by a foreigner. If incorrect, my apologies in advance.
Mary